Recovering a caustic solution via calcium carbonate crystal aggregates

ABSTRACT

Techniques for growing crystalline calcium carbonate solids such that the crystalline calcium carbonate solids include a volume of 0.0005 mm3 to 5 mm3, include a slaker to react quicklime (CaO) and a low carbonate content fluid to yield a slurry of primarily slaked lime (Ca(OH)2); a fluidized-bed reactive crystallizer that encloses a solid bed mass and includes an input for a slurry of primarily slaked lime, an input for an alkaline solution and carbonate, and an output for crystalline calcium carbonate solids that include particles and an alkaline carbonate solution; a dewatering apparatus that includes an input coupled to the crystallizer and an output to discharge a plurality of separate streams that each include a portion of the crystalline calcium carbonate solids and alkaline carbonate solution; and a seed transfer apparatus to deliver seed material into the crystallizer to maintain a consistent mass of seed material.

CROSS-REFERENCE TO RELATED APPLICATION

The application is continuation application of and claims priority toU.S. application Ser. No. 15/370,620, filed on Dec. 6, 2016, which is adivisional of, and claims priority to, U.S. patent application Ser. No.14/281,430, entitled “Recovering a Caustic Solution Via CalciumCarbonate Crystal Aggregates,” filed on May 19, 2014, now U.S. Pat. No.9,637,393, the entire contents of which are incorporated by referenceherein.

TECHNICAL FIELD

The disclosure is related to a method of recovering a caustic solutionfrom a carbonate solution.

BACKGROUND

The industrial Kraft process takes wood and converts it into wood pulpfor many uses. In general, the process involves cooking the wood chipsin chemicals, mainly comprising a mix of sodium hydroxide and sodiumsulfide, known commonly as white liquor in the pulping industry. Afterthe cooking process, the cooked wood is separated from the liquids, theresulting liquid is commonly called black liquor, with varying chemicalcomposition depending upon the type of wood. The black liquor isconverted back into white liquor in a process commonly known as acaustic-recovery process, or chemical recovery process.

The first step in the conventional caustic recovery process isconcentration of black liquor from the pulping process. Concentratedblack liquor is sent to a recovery boiler to (1) recover the keychemical compounds, such as sodium carbonate, sulfides etc.; (2) combustthe organics material in the black liquor; and (3) to recover energywhich may be used throughout the pulp and paper mill or exported out ofthe plant. The smelt from the recovery boiler is generally mixed with asolution commonly known as weak liquor from the caustic recoveryprocess, the resulting mixture of the weak liquor and the boiler smeltis commonly referred to as green liquor and generally contains sodiumcarbonate, sodium hydroxide, sodium hydrosulfide and may contain othercompounds such as sodium sulfite, sodium thiosulfate and other processor non-process impurities.

The green liquor and calcium oxide, CaO, solids (commonly known asquicklime or burnt lime) from the downstream calciner are fed into astirred tank reactor, generally known as slaker or lime-slaker. Tworeactions, reaction (1) and reaction (2), take place in the slaker.First CaO reacts with water in the green liquor to form calciumhydroxide (Ca(OH)₂, commonly known as slaked lime, hydrated lime,builders' lime, pickling lime, or Chuna) via reaction (1).

CaO_((s))+H₂O_((aq))→Ca(OH)_(2 (s))  (1)

As soon as any calcium hydroxide Ca(OH)₂ is formed it begins reactingwith sodium carbonate in the green liquor to form solid calciumcarbonate (CaCO₃) through nucleation, via reaction (2).

Ca(OH)₂(s)+Na₂CO₃(aq)⇄CaCO₃(s)+2NaOH(aq)   (2)

Reaction (1) and Reaction (2) are generally known as the slakingreaction and the causticization reaction, respectively; and bothreactions occur simultaneously anytime water containing carbonate ismixed with quicklime.

The bulk of the causticization reaction takes place in the slaker.Generally, the contents from the slaker are fed into a series of stirredtanks, typically referred to as causticizers, where the reactions areallowed to proceed to near completion. The resulting solution isreferred to as unclarified white liquor and contains, among otherchemicals, suspended CaCO₃ particles, called lime mud which are around15 μm in diameter.

Thereafter, generally the lime mud is first removed from the whiteliquor via clarifying tanks or pressurized filters. Typical filtrationequipment for this step includes pressurized tubular filters orpressurized disc filters. The resulting clarified white liquor isreturned to the pulping process to cook more wood chips, and the limemud is sent for further washing and filtration before being calcined.Calcination is the term for converting the lime mud (CaCO₃) intoquicklime (CaO):

CaCO₃(s)→CaO(s)+CO₂(g)   (3)

The resulting off-gas is typically cleaned and discharged to atmospherewhile the produced CaO is sent back to the slaker for reaction with thegreen liquor.

The lime mud in the conventional caustic recovery process fouls both thecalciner and any downstream gas processing equipment. Rotary kilns havebeen shown to tolerate the fouling caused by the lime mud and operatecontinuously requiring only minor shutdowns for cleaning andmaintenance. As a result, the rotary kiln is the most common type ofcalciner applied today to calcine lime mud and the hot off-gases fromthe kiln are commonly used to dry incoming lime mud as they will foulany other type of equipment. The rotary kiln is a large, expensive,difficult to operate piece of equipment and the off-gases are vented tothe atmosphere still containing a large quantity of high grade heatresulting in an overall thermal efficiency of around 40%. Many of thechallenges in the calcination section of the conventional causticrecovery process are a direct result of the fine particle size of limemud and its tendency to foul high temperature solids processingequipment.

SUMMARY

In a general implementation, a method for growing crystalline calciumcarbonate solids in the presence of an alkaline carbonate solution in afluidized-bed reactive crystallizer such that each of at least a portionof the crystalline calcium carbonate solids reach a volume of 0.0005 mm³to 5 mm³ includes: reacting, in a slaking process, quicklime (CaO) and alow carbonate content fluid to yield a slurry of primarily slaked lime(Ca(OH)₂); introducing the slurry of primarily slaked lime and analkaline solution including between 0.1M to 4.0M hydroxide and between0.1M to 4.1M carbonate into a fluidized-bed reactive crystallizer thatincludes a solid bed mass; reacting the Ca(OH)₂ slurry and the alkalinecarbonate solution to deposit a portion of the precipitated calciumcarbonate (CaCO₃) onto the existing bed of solids that (1) causes thesolids to grow in volume, (2) decreases a concentration of dissolvedcarbonate, and (3) increases a concentration of dissolved hydroxide; anddischarging a portion of the crystalline calcium carbonate solids andthe alkaline carbonate solution, the solids including particles thateach include a volume within the range from about 0.0005 mm³ to about 5mm³.

A first aspect combinable with the general implementation furtherincludes introducing seed material to maintain a constant mass of seedmaterial within the fluidized-bed reactive crystallizer.

In a second aspect combinable with any of the previous aspects, theCa(OH)₂ slurry content includes a range of percent by weight of solidCa(OH)₂ between: 2 wt % to 40 wt %; 20 wt % to 40 wt %; 25 wt % to 40 wt%; 30 wt % to 38 wt %; or 30 wt % to 35 wt %.

A third aspect combinable with any of the previous aspects furtherincludes controlling an environment of the slaking process to yieldCa(OH)₂ particles that are each sized between 0.1 to 100 micrometers,between 0.1 to 50 micrometers, between 0.1 to 20 micrometers, between0.1 to 10 micrometers, or between 0.1 to 5 micrometers.

In a fourth aspect combinable with any of the previous aspects, the lowcarbonate content fluid includes water, and an amount of carbonate inthe water includes less than about 0.1 moles of carbonate for every 1mole of CaO delivered to the slaking process.

In a fifth aspect combinable with any of the previous aspects,introducing the slurry of primarily slaked lime includes injecting theslurry into a recirculation stream influent upstream of thefluidized-bed reactive crystallizer.

In a sixth aspect combinable with any of the previous aspects,introducing the alkaline solution includes injecting the alkalinesolution into the recirculation stream influent upstream of thefluidized-bed reactive crystallizer.

In a seventh aspect combinable with any of the previous aspects,introducing the Ca(OH)₂ slurry includes feeding the slurry into one ormore planes that are orthogonal to fluid flow along a height of thefluidized-bed reactive crystallizer, and at one or more points withineach plane.

In an eighth aspect combinable with any of the previous aspects,introducing the Ca(OH)₂ slurry includes using an eductor to dilute theslurry with fluid and facilitate transport of slurry into thefluidized-bed reactive crystallizer with at least one injection port orat least one lance fluidly coupled to the fluidized-bed reactivecrystallizer.

A ninth aspect combinable with any of the previous aspects furtherincludes introducing an additive.

A tenth aspect combinable with any of the previous aspects furtherincludes lowering a viscosity of the Ca(OH)₂ slurry based on introducingthe additive.

An eleventh aspect combinable with any of the previous aspects furtherincludes reducing an amount of time to complete the slaking processbased on introducing the additive.

In a twelfth aspect combinable with any of the previous aspects, eachCa(OH)₂ particle in the slurry includes a volume within the range frombetween 5×10−10 mm3 and 5×10−4 mm3, between 5×10−10 mm3 and 6.5×10−5mm3, between 5×10−10 mm3 and 4×10−6 mm3, between 5×10−10 mm3 and 5×10−7mm3, or between 5×10−10 mm3 and 6.5×10−8 mm3, based on introducing theadditive.

In a thirteenth aspect combinable with any of the previous aspects, theadditive includes an inorganic or organic additive.

In a fourteenth aspect combinable with any of the previous aspects, theadditive includes a natural or synthetic additive.

A fifteenth aspect combinable with any of the previous aspects furtherincludes controlling a dissolution rate of Ca(OH)₂ in the fluidized-bedreactive crystallizer based on introducing the additive.

In a sixteenth aspect combinable with any of the previous aspects, theadditive includes nitrilotriacetic acid, phenol, sucrose, NH4Cl, orH-EDTA.

A seventeenth aspect combinable with any of the previous aspects furtherincludes accelerating a growth rate of the calcium carbonate crystalsformed on the solid bed mass within the fluidized-bed reactivecrystallizer, at a particular calcium saturation level, based onintroducing the additive.

An eighteenth aspect combinable with any of the previous aspects furtherincludes controlling one or more physical properties of the calciumcarbonate crystals formed within the fluidized-bed reactive crystallizerbased on introducing the additive.

A nineteenth aspect combinable with any of the previous aspects furtherincludes decreasing a porosity of the calcium carbonate crystals basedon introducing the additive.

A twentieth aspect combinable with any of the previous aspects furtherincludes increasing a hardness or a resistance to crushing of thecalcium carbonate crystals based on introducing the additive.

A twenty-first aspect combinable with any of the previous aspectsfurther includes reducing a presence of low melting salts associatedwith the calcium carbonate crystals based on introducing the additive,the low melting salts located on a surface of the calcium carbonatecrystal aggregates, within a lattice of the calcium carbonate crystals,or within pores of the calcium carbonate crystal aggregates.

A twenty-second aspect combinable with any of the previous aspectsfurther includes reducing a spontaneous nucleation of new calciumcarbonate crystals based on introducing the additive.

In a twenty-third aspect combinable with any of the previous aspects,the height of the solid bed mass is between 15 feet and 50 feet, between30 feet and 50 feet, or between 30 feet and 40 feet.

A twenty-fourth aspect combinable with any of the previous aspectsfurther includes controlling a level of total suspended solids withinthe fluidized-bed reactive crystallizer to between 0 ppm and 10,000 ppm,between 50 ppm and 5,000 ppm, or between 100 ppm and 1500 ppm.

In a twenty-fifth aspect combinable with any of the previous aspects,controlling a level of total suspended solids within the fluidized-bedreactive crystallizer includes at least one of: using a solids-liquidseparation process; controlling a calcium loading rate; or controllingat least one equipment configuration characteristic.

In a twenty-sixth aspect combinable with any of the previous aspects,the equipment configuration characteristic includes at least one of asolid mass bed height, or a chemical injection delivery device.

In a twenty-seventh aspect combinable with any of the previous aspects,the solids-liquid separation process includes at least one offiltration, clarification, or centrifugation.

A twenty-eighth aspect combinable with any of the previous aspectsfurther includes minimizing a time spent outside of the solid bed massof any fluid including undissolved Ca(OH)₂ which is withdrawn from andintended to be returned to the fluidized-bed reactive crystallizer.

In a twenty-ninth aspect combinable with any of the previous aspects, avolume of each discharged crystalline calcium carbonate solid is between0.065 mm³ to 4.2 mm³, between 0.22 mm³ to 1.77 mm³, or between 0.32 mm³to 1.02 mm³.

In a thirtieth aspect combinable with any of the previous aspects, avolume of each discharged crystalline calcium carbonate solid is between0.0005 mm³ to 0.04 mm³.

A thirty-first aspect combinable with any of the previous aspectsfurther includes separating a portion of the discharged crystallinecalcium carbonate solids that are below a desired volume; and returningthe separated portion to the fluidized bed reactive crystallizer tocontinue to grow.

A thirty-second aspect combinable with any of the previous aspectsfurther includes removing, from the alkaline carbonate solution, anamount of carbonate between 10 mole % to 100 mole %, between 15 mole %to 50 mole %, between 15 mole % to 40 mole %, or between 20 mole % to 30mole % that is delivered to the fluidized-bed reactive crystallizer aspart of the influent alkaline solution stream.

In a thirty-third aspect combinable with any of the previous aspects,the removed carbonate includes a solid calcium carbonate.

In a thirty-fourth aspect combinable with any of the previous aspects,removing, from the alkaline carbonate solution, an amount of carbonateincludes leaving an amount of hydroxide from the solid Ca(OH)₂ influentslurry stream dissolved in the alkaline carbonate solution as an aqueoushydroxide.

In a thirty-fifth aspect combinable with any of the previous aspects,the seed material possesses a similar crystalline structure to that ofthe crystalline calcium carbonate solids, and the crystalline structureis similar to at least one of silica, aragonite, calcite, or vaterite.

In a thirty-sixth aspect combinable with any of the previous aspects, avolume of each seed in the seed material is selected such that the seedmaterial makes up between 0.5 wt % to 20 wt %, between 1 wt % to 10 wt%, between 2 wt % to 7 wt %, or between 2 wt % to 5 wt % of thedischarged crystalline calcium carbonate solids.

A thirty-seventh aspect combinable with any of the previous aspectsfurther includes processing at least a portion of the crystallinecalcium carbonate solids to produce a seed material; and growing newcalcium carbonate crystal aggregates on the seed material.

A thirty-eighth aspect combinable with any of the previous aspectsfurther includes controlling material introduced into the fluidized-bedreactive crystallizer to control an impurity concentration level withinthe crystalline calcium carbonate solids below a maximum acceptableimpurity concentration level.

In a thirty-ninth aspect combinable with any of the previous aspects, animpurity of the crystalline calcium carbonate solids includes one ormore of phosphates, calcium carbonate, magnesium ions, Group II A ions,strontium, radium, iron ions, phosphonates, or zinc.

In a fortieth aspect combinable with any of the previous aspects,reacting, in a slaking process, quicklime (CaO) and a low carbonatecontent fluid to yield a slurry of primarily slaked lime (Ca(OH)₂)includes reacting, in a slaking process for a particular time and at aparticular temperature that are based at least in part on a desired rateof conversion of calcium oxide to calcium hydroxide.

In a forty-first aspect combinable with any of the previous aspects, theparticular time includes between 1 minute and 120 minutes.

In a forty-second aspect combinable with any of the previous aspects,introducing the slurry of primarily slaked lime includes injecting, withone or more lances, the slurry directly into the solid bed mass.

In a forty-third aspect combinable with any of the previous aspects, theone or more lances includes a plurality of lances positioned to injectthe slurry at injecting sites through a horizontal cross section of anarea of the solid bed mass.

In a forty-fourth aspect combinable with any of the previous aspects,introducing an additive includes introducing an additive into theslaking process or the Ca(OH)₂ slurry transfer system.

A forty-fifth aspect combinable with any of the previous aspects furtherincludes increasing a transfer rate of the slurry to the fluidized-bedreactive crystallizer based on introducing the additive.

Another general implementation includes a system for growing crystallinecalcium carbonate solids in the presence of an alkaline carbonatesolution such that each of at least portion of the crystalline calciumcarbonate solids include a volume of 0.0005 mm³ to 5 mm³, including aslaker configured to react quicklime (CaO) and a low carbonate contentfluid to yield a slurry of primarily slaked lime (Ca(OH)₂); afluidized-bed reactive crystallizer that encloses a solid bed mass andincludes an input for a slurry of primarily slaked lime, an input for analkaline solution including between 0.1M to 4.0M hydroxide and between0.1M to 4.1M carbonate, and an output for crystalline calcium carbonatesolids that include particles each with a volume between 0.0005 mm³ andabout 5 mm³ and an alkaline carbonate solution; a dewatering apparatusthat includes an input coupled to the output of the fluidized-bedreactive crystallizer and an output configured to discharge a pluralityof separate streams that each include a portion of the crystallinecalcium carbonate solids and alkaline carbonate solution; and a seedtransfer apparatus configured to deliver seed material into thefluidized-bed reactive crystallizer to maintain a consistent mass ofseed material within the fluidized-bed reactive crystallizer.

In a first aspect combinable with the general implementation, the seedtransfer apparatus is further configured to return a portion of thecrystalline calcium carbonate solids that include a volume less thanabout 0.3 mm³ from the dewatering apparatus, together with the seedmaterial, to the fluidized-bed reactive crystallizer.

In a second aspect combinable with any of the previous aspects, the seedtransfer apparatus is further configured to return a portion of thecrystalline calcium carbonate solids that include a volume less thanabout 0.3 mm³ from the dewatering apparatus, to the fluidized-bedreactive crystallizer in separate batches from the seed material.

In a third aspect combinable with any of the previous aspects, thefluidized-bed reactive crystallizer includes a spouted bed.

In a fourth aspect combinable with any of the previous aspects, thefluidized-bed reactive crystallizer further includes a cone shaped entrysection.

In a fifth aspect combinable with any of the previous aspects, the inputfor a slurry of primarily slaked lime includes an injection port fluidlycoupled to a recirculation stream influent upstream of the fluidized-bedreactive crystallizer.

In a sixth aspect combinable with any of the previous aspects, the inputfor the alkaline solution includes an injection port fluidly coupled tothe recirculation stream influent upstream of the fluidized-bed reactivecrystallizer.

In a seventh aspect combinable with any of the previous aspects, theinput for a slurry of primarily slaked lime includes a plurality ofinjection ports positioned to introduce the slurry into one or moreplanes that are orthogonal to fluid flow along the height of thefluidized-bed reactive crystallizer and at one or more points withineach plane.

An eighth aspect combinable with any of the previous aspects furtherincludes an input for an additive.

In a ninth aspect combinable with any of the previous aspects, theheight of the solid bed mass is between 15 feet and 50 feet, between 30feet and 50 feet, or between 30 feet and 40 feet.

A tenth aspect combinable with any of the previous aspects furtherincludes a solids-liquid separator to control a level of total suspendedsolids within the fluidized-bed reactive crystallizer.

In an eleventh aspect combinable with any of the previous aspects, thesolids-liquid separator includes at least one of a filter, aclarification system, or a centrifuge.

A twelfth aspect combinable with any of the previous aspects furtherincludes a controller configured to control a calcium loading rate tocontrol a level of total suspended solids within the fluidized-bedreactive crystallizer.

A thirteenth aspect combinable with any of the previous aspects furtherincludes a separator fluidly coupled to an output of the dewateringapparatus to separate a portion of the discharged crystalline calciumcarbonate solids that are below a desired volume, the separatorincluding an output fluidly coupled to the fluidized bed reactivecrystallizer to return the separated portion to the fluidized bedreactive crystallizer.

In a fourteenth aspect combinable with any of the previous aspects, thefluidized bed reactive crystallizer is further configured to process atleast a portion of the crystalline calcium carbonate solids to produce aseed material, and grow new calcium carbonate crystal aggregates on theseed material.

In a fifteenth aspect combinable with any of the previous aspects, theinput for the slurry of primarily slaked lime includes an eductorconfigured to dilute the slurry with fluid and facilitate transport ofslurry into the fluidized-bed reactive crystallizer; and at least oneinjection port or at least one lance fluidly coupled to thefluidized-bed reactive crystallizer.

Various implementations of the systems and processes for recovering acaustic solution from a carbonate solution may include one or more ofthe following features. For example, the disclosed systems and processesmay utilize a fluidized-bed reactive crystallizer rather than a standardcrystallizer vessel. As another example, systems and processes mayfeature the production of calcium carbonate crystal aggregates withaverage volumes equivalent to spheres with diameters between 0.1 mm to 2mm. As another example, systems and processes may feature calciumcarbonate crystal aggregates having the size, morphology, and physicalproperties so that any moisture they retain upon removal from a solutionis primarily surface moisture and easily displaced or removed. Asanother example, systems and processes may features a low carbonateconversion rate, tolerance for TSS in influent streams, high solution pHand slow dissolution of Ca(OH)₂ slurry. As a further example, systemsand processes may run at higher effluent TSS levels than are normallyexpected to produce poor reactor performance in lower pH (e.g., pH 7-10)systems, because it is typically expected that a higher TSS in theeffluent is associated with a reduction in the percent of calciumretained on the pellets inside the bed mass. Further, the disclosedsystems and processes may achieve economically viable retention rates atcalcium loading rates comparable to water treatment industries. In someexample implementations, the disclosed systems and processes may operatewith high pH (for example, pH>14) where most of the CO₂ is present ascarbonate (CO₃), and very little as bicarbonate (HCO₃). Further, thesystems and processes may utilize a spouted bed or cone fluidized-bedreactor design for an elegant and less expensive solution that is bettersuited for direct feed of a concentrated lime slurry when compared tosystems designed to deal with chemical addition in applications wherethe chemical dissolution rates are much faster and where relativecalcium carbonate supersaturation is more likely to occur.

These general and specific aspects may be implemented using a device,system, process, or any combinations of devices, systems, or processes.For example, a system of one or more computers can be configured toperform particular actions by virtue of having software, firmware,hardware, or a combination of them installed on the system that inoperation causes or cause the system to perform the actions. One or morecomputer programs can be configured to perform particular actions byvirtue of including instructions that, when executed by data processingapparatus, cause the apparatus to perform the actions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example implementation of a fluidized-bed reactivecrystallization process unit that may be used as part of a largerprocess for growing calcium carbonate crystal aggregates while reducingthe dissolved carbonate content and increasing the dissolved hydroxidecontent of the effluent liquid stream;

FIG. 2 illustrates a second example implementation of a fluidized-bedreactive crystallization process unit that may be used as part of alarger process for growing calcium carbonate crystal aggregates whilereducing the dissolved carbonate content and increasing the dissolvedhydroxide content of the effluent liquid stream;

FIG. 3 illustrates a third example implementation of a fluidized-bedreactive crystallization process unit that may be used as part of alarger process for growing calcium carbonate crystal aggregates whilereducing the dissolved carbonate content and increasing the dissolvedhydroxide content of the effluent liquid stream;

FIG. 4 illustrates an example process for growing calcium carbonatecrystal aggregates while reducing the dissolved carbonate content andincreasing the dissolved hydroxide content of the effluent liquidstream;

FIG. 5 illustrates an example process for growing and processing calciumcarbonate crystal aggregates to convert clarified green liquor from apulp plant to clarified white liquor; and

FIG. 6 illustrates an example process for growing calcium carbonatecrystal aggregates while reducing the dissolved carbonate content andincreasing the dissolved hydroxide content of the effluent liquid in anair-capture plant, in association with an air-fired or oxygen-firedcalciner, and CO₂ capture, and gas absorber.

DETAILED DESCRIPTION

The systems and processes described in the present disclosure, in someimplementations, segregates the reactions (1) and (2) mentioned aboveinto different units within the processes instead of allowing thesereactions to occur within a common environment and/or unit. The systemsand processes described in the present disclosure also use a differentprocess and apparatus, within which reaction (2) takes place, ascompared to convention systems and processes. This apparatus, afluidized-bed reactive crystallizer (pellet reactor), enables theproduction of large, low porosity pellets that are easier to handle anddewater than the above mentioned lime mud.

This pellet reactor technology is currently used in applications withinthe water softening and water treatment industries under significantlydifferent conditions, environments, chemistry and equipmentconfigurations, and for different end results, than as it is applied inthe systems and processes described here. For example, the mainapplications of the pellet reactor within the water softening and watertreatment industries are to remove trace amounts of phosphates and/ormetals from water. The pellets themselves are not considered as theprimary product, but rather as a by-product. The pellet reactorconfiguration needed to accomplish these water softening and treatmentactions includes complex chemical injection systems, or lances withdistribution nozzles, to ensure proper distribution of the chemistry andavoidance of localized supersaturation and the associated problems. Thepellet reactor bed heights are also significantly different for watertreatment and softening processes as compared to the systems andprocesses described herein, typically ranging between 20 feet to 30feet. Further, while the current applications of pellet reactorequipment do sometimes use calcium hydroxide solutions within theirchemistry, they do not have restrictions on water input to their systemsand as such they can use much more dilute sources of calcium hydroxidesolutions (e.g., typically less than 20 weight percent, often less than10 weight percent Ca(OH)₂).

FIG. 1 illustrates an example implementation of a fluidized-bed reactivecrystallization process unit (e.g., also referred to herein as afluidized-bed reactive crystallizer, a fluidized-bed reactor, orreactor) that may be used as part of a larger process for recovering acaustic solution from a carbonate solution. In some implementations, theillustrated fluidized-bed reactive crystallization process unit may beused as unit 1120 as shown and described in the process of FIGS. 4-6.The fluidized-bed reactive crystallization process grows calciumcarbonate crystal aggregates while reducing the dissolved carbonatecontent and increasing the dissolved hydroxide content of the effluentliquid stream. The dissolved carbonate content can be reduced usingother calcium based salts such as calcium chloride, however in thesecases, the solution's hydroxide concentration would not increase.

In some aspects, the solids (e.g., crystal aggregates) produced consistof calcium carbonate in the form of calcite, aragonite, or vaterite. Asillustrated in FIG. 1, the example system 100 and process includes areactor 108, a fluidized bed of solid material 107, influent streams 101and 103 that enter at or near the bottom of the reactor 108, influentseed material 4, influent stream 2 containing calcium hydroxide slurry,effluent stream 105 of calcium carbonate crystal aggregates material,effluent alkaline solution stream 106, entry points 109 used to eitheradd supplemental chemicals such as additives or additional calciumhydroxide slurry or for sample removal, and a control system orcontroller 110.

Controller 110 (also shown in FIGS. 2-3), in some aspects, may becommunicably coupled to one or more apparatus (e.g., valves, pumps, flowmeters, sensors, or otherwise) used to control the system 100 andassociated processes performed with or by the system 100. The controller110 can include digital electronic circuitry (e.g., a PLC controller),or computer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, orcombinations of one or more of them. For example, the controller 110 canbe a microprocessor based controller as well as an electro-mechanicalbased controller. Instructions and/or logic in the control system (e.g.,to control the system 100), can be implemented as one or more computerprograms, e.g., one or more modules of computer program instructions,encoded on computer storage medium for execution by, or to control theoperation of, data processing apparatus. Alternatively or in addition,the program instructions can be encoded on an artificially generatedpropagated non-transitory signal, e.g., a machine-generated electrical,optical, or electromagnetic signal that is generated to encodeinformation for transmission to suitable receiver apparatus forexecution by a data processing apparatus. In some instances, thecontroller 110 may be a slave to the control system 1108 described withreference to FIGS. 4-6.

The entry points 109 may be positioned at one or more points along aplane orthogonal to the direction of fluid flow through the reactor 108,such that proper mixing of the additive or calcium hydroxide may beenhanced and/or achieved. These entry points 109 may include, in someimplementations, injection lances and nozzles that are designed toenhance the mixing and distribution of the chemical or additive withinthe fluidized bed mass 107 and may also include or be coupled to aneductor to assist with transfer of the additive or chemical from theinjection system into the solid bed mass. In some aspects, there may bemultiple injection lances or nozzles that extend from various locationsaround a circumference (or other exterior surface) of the reactor 108.The multiple injection lances or nozzles may extend into the reactor 108in parallel or at various angles. In some cases, one or more of thelances or nozzles may include a main trunk and branches that angularlyextend from the trunk within the reactor 108.

In some example implementations described herein, a fluidized-bedreactive crystallizer may include one, some, or all of the followingfeatures: (1) utilizes liquid flow, in an upward direction to suspendsolid particles that make up the solid bed mass component of thefluidized-bed; (2) partially filled with a suitable seed material thatserves as the surface area on which calcium carbonate can precipitate togrow the solid particle bed mass; (3) is fed seed material in acontinuous or batch mode in such a way as to replace seed material lostfrom the apparatus when full sized solid particles are discharged andsent to downstream processing; and/or (4) is fed a slurry containingCa(OH)₂ that provides the driving force for precipitating the calciumcarbonate onto the solid bed.

The first step in this example process shown in FIG. 1 is to introducethe solid seed material influent stream 104, which is used to providesurface area for the precipitation of calcium carbonate. In some exampleimplementations described herein, seed material 104 may be or mayinclude solid particles each having a volume less than the target volumefor a discharged pellet. For example, the fluidized-bed reactivecrystallizer may be initially filled with seed material 104, and theprecipitated calcium carbonate deposits on this seed material 104,causing the volume of each particle to increase until it reaches thedesired size range and is discharged from the bed 107. The dischargedcalcium carbonate crystal aggregates that leave the fluidized-bedreactive crystallizer 108 may contain a portion of seed material 104.

In some aspects, the seed material 104 may include limestone thatcontains calcium carbonate in the form of calcite, aragonite, and/orvaterite. In some aspects, the seed material 104 used contains silica.In some aspects, the seed material 104 is obtained from other industrialsources such as pellets (e.g., crystal aggregates) produced in wastewater treatment processes or water softening processes, that has beenground to the desired seed size. In some aspects, the seed material isobtained by extracting a portion of the discharged mature calciumcarbonate crystal aggregates, grinding them down to seed size andre-using as seed material. This may help maintain consistency within thepellet material instead of introducing new material from an externalsource in the form of seed.

In some aspects, a designated pellet reactor may be used to produce seedmaterial in cases where there may be issues with composition (e.g.,compositional variance, impurities) of the calcium carbonate seednormally procured from a commercial vendor, for example a limestonevendor that provides ground limestone from geological source(s). In someexamples, sufficient seed material 104 is added such that, oncefluidized, the solid bed 107 may expand to have a height within therange of 15 feet to 50 feet.

In a next step, the alkaline solution 1 is then fed to the reactor 108.The influent alkaline solution stream 1 and a recirculation stream 3,which takes solution from the top of the fluidized-bed reactivecrystallizer 108 and feeds it back into the bottom of the reactor 108,may enter the reactor 108 independently and may be used to provide aproper fluidization of the solid bed material 107. In some aspects, theprocess may be configured with a high recirculation flow to influentflow ratio range of approximately 19:1 to 50:1, where the recirculationflow provides the bulk of the flow which fluidizes the solid bed mass107.

In some example implementations described herein, a solid bed mass mayinclude solid particles that are fluidized within the reactor 108. Thebulk of this solid mass may remain within the reactor 108, with only asmall amount being withdrawn on a continuous or semi-continuous basis asmature pellets (e.g., mature calcium carbonate crystal aggregates). Thesolid bed mass, in some aspects, may provide a surface area onto whichthe calcium carbonate produced from the causticization reaction canprecipitate. The majority of particles within the solid bed mass may beof size similar to the seed size or larger.

The fluidized bed of solids 107 may have excellent mixing in thedirections orthogonal to the average liquid flow direction and intimatecontact between the solids, which make up the bed 107, and the upwardflow of fluid may keep the solids suspended. The velocity of the upwardfluid flow is chosen based on the desired particles sizes to befluidized and will only fluidize particles within a certain range. Thefluidization flow is used to promote mixing (reduce calcium carbonatesupersaturation) and control bed density, keep bed density high enoughto maximize exposure of the solution to the surface area of the bed massand prevent loss of bed material from reactor 108, and keep bed densitylow enough to minimize bed mass attrition and/or agglomeration issues.

In some aspects, the fluidization velocity range is between 60 metersper hour (m/h) to 120 m/h and in some aspects between 80 m/h and 100 m/hfor mature pellet diameter range of between about 0.85 mm and 1.2 mm. Insome aspects the fluidization velocity range is between 2 m/h and 70 m/hfor a mature pellet diameter range between about 0.1mm and 0.4mm and insome examples between 2 m/h and 20 m/h for a mature pellet diameterrange of about 0.1 mm and 0.2 mm.

In some example implementations, a fluidization velocity may define alinear velocity at which the fluid travels in a generally upwarddirection through the fluidized-bed reactive crystallizer. Fluidizationvelocity is sometimes used to help define the range of operatingconditions that provide an optimum bed density or expansion of pelletbed such that the bed density is not too high as to cause abrasion andultimately collapse of the bottom section of the bed (e.g., the bottomsection of the fluidized bed where a large portion of the mature pelletstend to reside). It may also provide an optimum bed density that is nottoo low so as to reduce the retention of calcium onto the pellets and/orincrease the amount of calcium turning into fines.

If the particles are too large, then the upward flow of fluid will notbe able to suspend them and they will settle out of the fluidized bedand rest at the bottom of the unit and if the particles are too smallthey will be carried out of the fluidized bed through stream 106. In thecase of a desired particle diameter of 1 mm, particles which are muchlarger, such as 5 mm in diameter, will settle out while small particlessuch as those produced by nucleation will be carried away in stream 106.In some aspects, the target diameter of the discharged mature pellet isabout 0.85 mm to 1.2 mm.

In some aspects, there is a pumping energy associated with properfluidization of this size of pellet in the alkaline solution being usedfor fluidization. In some aspects, the target mature pellet diameter issmaller, at about 0.100 mm to 0.85 mm diameter or at about 0.100 mm to0.400 mm diameter. In some aspects, a smaller final pellet size mayfacilitate maintenance of the bed density or bed expansion in thissystem at the same optimal range as seen in the previous system (e.g., asystem as described above that produces larger mature pellet sizes)would take less pumping energy but would still provide pellets of a sizethat can be efficiently dewatered.

In some example implementations, the bed expansion may define a percentincrease in bed volume between when there is no fluidization (e.g.,pellet bed is static and effectively resting at the bottom of thefluidized-bed reactive crystallizer) and when the system is beingfluidized such that there is more fluid between the pellets and the bedvolume has expanded, leading to a higher bed level in the fluidized-bedreactor 108.

Once proper fluidization of bed material is established and the heightof the fluidized solid bed mass is between 15 feet and 50 feet withinthe reactor 108, the influent stream 102 containing calcium hydroxideslurry is independently added to the reactor 108, at one or more entrypoints, in a controlled manner, normally within a range between 5kg-Ca/m²/hr and 35 kg-Ca/m²/hr, and in some aspects within a rangebetween 20 k-Cag/m²/hr and 30 kg-Ca/m²/hr, such that spontaneousnucleation (e.g., production of smaller particles than desired) isinhibited while achieving a high rate of pellet growth.

In some aspects, this process may include calcium hydroxide slurrieswithin the concentration range between approximately 2 wt % andapproximately 40 wt %. The carbonate to calcium molar ratio entering thereactor 108 is usually in the range 1.1:1 to 30:1. The carbonate in thealkaline stream may, in some aspects, be in molar excess of the calciumhydroxide such that there is carbonate remaining in the effluent streamleaving the fluidized-bed reactive crystallizer. The calcium hydroxidedissolves into solution and becomes the limiting reagent in reaction(2), where calcium reacts with the dissolved carbonate content broughtin by influent streams 1, 102, and 3 in close proximity to the solid bedmaterial 107. This facilitates the precipitation of calcium carbonateonto the solid bed mass surface, resulting in growth of the individualsolid particles. Example implementations of systems and/or processesdescribed herein may work with a very low conversion rate of carbonatespecies across the bed. For example, example implementations of systemsand/or processes described herein can be operated with approximately10-25% conversion of aqueous carbonate to CaCO₃ solids.

The solution moving through the reactor 108 may be at a pH range above12, and in some aspects above 14. This high pH inhibits the dissolutionof the calcium hydroxide entering the reactor 108. Calcium hydroxide isknown to have a non-instantaneous dissolution rate in alkaline solutionsof much lower pH such as 7 to 10 as seen in water softening and wastewater treatment applications; the higher pH of example implementationsof systems and/or processes described herein (e.g., pH of greater than14) further inhibit the dissolution rate of calcium hydroxide.

In some aspects, example implementations of systems and/or processesdescribed herein may handle the slow dissolution of calcium hydroxide byextending the solid bed height to between 15 feet and 50 feet, such thatthere is sufficient residence time within the bed mass to fully dissolvethe calcium hydroxide and react the calcium. This may promote calciumcarbonate precipitation onto the solid bed mass (calcium carbonatecrystal aggregates) instead of promoting nucleation outside of the solidbed which would result in spontaneous nucleation and fines production.

The Ca(OH)₂ entering the fluidized-bed reactive crystallizer 108 may bethe limiting reagent and as such, may create an upper limit for how muchinfluent carbonate is converted to calcium carbonate (and hence removedfrom solution). Therefore, the moles of influent Ca(OH)₂ may set amaximum difference in molar concentration of dissolved carbonate betweenthe influent and effluent streams. The moles of influent Ca(OH)₂ mayalso set a maximum difference in molar concentration of dissolvedhydroxide between the influent and effluent streams. In one example,where the influent [OH] is about 1M and the influent [CO₃] is about0.5M, the calcium loading rate from the Ca(OH)₂ lime slurry is about 15kg-Ca/m²/hr, the recirculation to influent flow ratio is about 19:1, thefluidization velocity is between 80 m/h and 100 m/h, then the resultingdifference (e.g., decrease) in molar concentration of carbonate betweenthe influent and effluent streams can be between about 0.07M to 0.10M,which is between about 0.0037M to 0.0053M per bed pass. The associateddifference (e.g., increase) in molar concentration of hydroxide betweenthe influent and effluent streams can be between about 0.14M to about0.20M, which is between about 0.0074M to 0.0105M per bed pass. For everymole of carbonate precipitated from solution by reaction (2), there are2 moles of hydroxide dissolved into the solution.

Factors that can influence the molar concentration differences ofcarbonate and hydroxide, and the amount of Ca(OH)₂ reacted via reaction(2) across the fluidized-bed reactive crystallizer include flow rate ofinfluent, fluid velocity requirements of the fluidized-bed reactivecrystallizer 108, calcium loading rates, influent [OH] and [CO₃], andmechanical configuration of the fluidized-bed reactive crystallizer 108.

As a result of controlling the rate of reaction (2) and the environmentwhere reaction (2) occurs, calcium carbonate crystal aggregates, eachhaving an average volume equivalent to spheres with diameters between0.1 mm and 2 mm, may be produced. While these solid particles grow inmass, they move towards the bottom of the reactor 108. The liquid fromthis process has had a portion of the aqueous carbonate precipitated(e.g., removed from solution) and a proportional amount of hydroxide hasdissolved from the Ca(OH)₂ into the liquid and the resulting mixture isdischarged as the effluent stream 106 while the solid calcium carbonatecrystal aggregates which have grown to the desired size are removed fromthe fluidized bed reactive crystallizer 108 as effluent stream 5.

The larger individual pellet size, in conjunction with the low porosityof the resulting solid particles, may allow for simple downstreamdewatering processes. This may also result in a calcium carbonateproduct with a much lower moisture content of less than about 15% of thetotal weight of the calcium carbonate crystal aggregates (as measuredafter the solids are drained on a screen). The inlet ports 109 may beused to provide additional chemicals such as additives or calciumhydroxide slurry in order to optimize the reactor 108 environment forproducing pellets within the target size range, hardness, crushingstrength, porosity, composition and purity, and to minimize spontaneousnucleation and production of particles smaller than desired and/orimpurities from entering the effluent product streams and downstreamprocessing steps.

FIG. 2 illustrates another example implementation of a fluidized-bedreactive crystallization process unit that may be used as part of alarger process for growing calcium carbonate crystal aggregates whilereducing the dissolved carbonate content and increasing the dissolvedhydroxide content of the effluent liquid stream. In someimplementations, the illustrated fluidized-bed reactive crystallizationprocess unit 208 may be used as unit 1120 as shown and described in theprocess of FIGS. 4-6. In some aspects, the illustrated fluidized-bedreactive crystallization process unit 208 may be substantially similarto the unit 108 shown in FIG. 1, with the exception that influentstreams 201, 202 and 203, in the example of FIG. 2, may be combineddirectly upstream of reactor 208 to facilitate the mixing of influentstream 202 such that plugging and local unfavorable concentrationconditions are minimized. Also, in some aspects, the entry section ofthe vessel in FIG. 2, where the influent streams are introduced, may becone-shaped to facilitate good mixing behavior of both solution andsolids.

When chemical addition to the fluidized-bed reactive crystallizer 208has a fast dissolution rate, it may mean that the chemical dissolvesinstantly. As seen in waste water and water softening applications offluidized-bed reactive crystallizers, this can lead to localizedsupersaturation at the entrance point(s) of the reactor 208, which, ifnot effectively mixed, can promote spontaneous nucleation, or in otherwords, produce fine particles (fines) which do not add to the pellet bedmass but instead are small enough to be elutriated by the upward flow ofliquid and leave the bed as fine material. This resulting fine materialreduces the reactor's crystallization efficiency and may requiredownstream filtration.

The configuration in FIG. 2 relies, at least in part, on the unmodified,slow dissolution rate of Ca(OH)₂ in the high pH environment of exampleimplementations of systems and/or processes described herein to allowfor mixing outside and upstream of the solid bed mass without nucleationoccurring which would result in the production of smaller than desiredparticles, commonly known as fines. Example implementations of systemsand/or processes described herein also employ the use of highconcentrations of Ca(OH)₂ slurry influent.

In some aspects, high slurry concentrations of above 30 wt % and up to40 wt % Ca(OH)₂ are used to minimize diluting the overall processsolution with excess water. There may be challenges to the use of thesehigh concentrations of lime slurry. For example, Ca(OH)₂ slurries above30 wt % Ca(OH)₂ are known to exhibit challenging fluid behaviors such aspseudo-plastic, bingham, and dilatant behavior. These behaviors can makemixing and transfer of this slurry difficult and prone to plugging,especially through the small lines required in the standard chemicallance injection systems.

The configuration shown in FIG. 2 may minimize (e.g., all or in part)these issues by eliminating the smaller line sizes and associated linerestrictions and obstructions that are present with the lance injectionequipment associated with FIG. 1. Because dissolution of Ca(OH)₂ may beslow, mixing with recirculation stream prior to contacting the solid bedmass will produce minimal production of total suspended solids (TSS)while facilitating efficient mixing and distribution of the Ca(OH)₂ inthe influent stream. This injection process may be less complicated thanuse of multiple small inside diameter, high velocity, chemical injectionlances needed to introduce the Ca(OH)₂ directly into the bottom of thesolid bed mass.

In some example implementations, TSS, or total suspended solids, mayinclude undissolved solids present in solution leaving and/or enteringthe solid bed mass, in the size range of about 2 microns to about 100microns, and which are typically carried out of the bed (e.g.,elutriated) by fluidization velocities that are within a range capableof suspending but not elutriating the larger calcium carbonate solid bedmass.

As shown, the system 200 may also differ from the system 100 based onthe entry location of influent stream 201, which is the alkalinesolution. This entry position is made possible by the nature of exampleimplementations of systems and/or processes described herein, where theoverall [CO₃] and [OH] in the influent stream 201 is not much differentthan that of the recirculation stream 203, and the recirculation streamis the bulk of the fluid entering the bottom of the reactor 208. Thisallows the insertion of the influent stream 201 into the recirculationstream directly upstream of the reactor entrance, rather thanindependently into the reactor 208, as it does not significantly impactthe [OH] and [CO₃] compositions within the reactor 208.

FIG. 3 illustrates another example implementation of a fluidized-bedreactive crystallization process unit that may be used as part of alarger process for growing calcium carbonate crystal aggregates whilereducing the dissolved carbonate content and increasing the dissolvedhydroxide content of the effluent liquid stream. In someimplementations, the illustrated fluidized-bed reactive crystallizationprocess unit 308 may be used as unit 1120 as shown and described in theprocess of FIGS. 4-6. In some aspects, the illustrated fluidized-bedreactive crystallization process unit 308 may be substantially similarto the unit 108 shown in FIG. 1, with the exception that that slipstream310 may be pulled from the recirculation loop and sent into an eductor311, where it combines with the influent lime slurry stream 302 andpushes the lime slurry through into the reactor 308. This system 300 mayresolve plugging issues seen when using Ca(OH)₂ slurry concentrationsbetween 20 wt % to 40 wt %, in particular above 30 wt %, withoutremoving the chemical lance injection systems, in that when using thelance injection system, the eductor 311 is added immediately upstream ofthe lance, as shown in FIG. 3. The eductor 311 utilizes a slipstream ofthe larger recirculation flow to dilute the solids in the lime slurryand/or help push the lime slurry through the lance system into thereactor 108. The slow dissolution rate of Ca(OH)₂ coupled with a shorttime for the fluid to travel from the eductor 311 into the fluidized bed307 may prevent (e.g., all or partially) the causticization reaction andnucleation from occurring and producing particles smaller than thedesired size (e.g., fines).

Previous lab scale testing of a fluidized-bed reactive crystallizerunder process conditions of approximately 0.5M to 1.5M hydroxide and 0.2to 0.8M carbonate (e.g., process conditions covered by the process shownin FIG. 1) used calcium hydroxide slurries containing as low as about 2wt % Ca(OH)₂ and as high as about 38 wt % Ca(OH)₂.

Another example of this process was tested in a fluidized-bed reactivecrystallizer under process conditions of approximately 0.5M to 1.5Mpotassium hydroxide and 0.2 to 0.8M potassium carbonate, (processconditions covered by this process), used calcium hydroxide slurriescontaining as low as about 3 5wt % Ca(OH)₂ and as high as about 40 wt %Ca(OH)₂.

In one example of this process carried out in a fluidized-bed reactivecrystallizer (e.g., as shown in FIG. 3), the pellets were grown up toapproximately 0.85 mm to 1.2 mm diameter in an alkaline environmentcontaining approximately 0.4M to 1.5M potassium hydroxide and 0.2M to0.9M potassium carbonate on calcite seeds procured from a watersoftening treatment process. The seed content was approximately 5-10 wt% of the mature pellet. The resulting mature calcite pellets producedand discharged from the fluidized-bed reactive crystallizer exhibitedproperties and compositions as described below.

In the above example, the packed bulk density of the calcium carbonatepellets was measured at about 1.63-1.68 g/cm³.

The particle size distribution (PSD) of a sample of mature pelletsdischarged from the above example of the process are shown in the tablebelow:

CaCO3 Pellet PSD info (Hazen) Size (Micron) Cumulative Passing % 141099.8 1190 99.7 1000 87 841 16.9 595 0.3 0 0.000001

Elemental composition of washed and dried calcium carbonate pelletsgrown from the above example of the process:

11743 HRI 53549 Washed and Dried 5/30/2013 16:52 Predicted Element Wt %Form Wt % Al 0.068 Al₂O₃ 0.128 Ca 40.8 CaCO₃ 102 Fe_(Total) 0.000Fe₂O_(3 Total) 0.000 K 0.568 KOH 0.815 Mg 0.232 MgO 0.385 Mn 0.000 MnO0.000 Na 0.064 Na₂O 0.086 P 0.006 P₂O₅ 0.014 Si 0.273 SiO₂ 0.584 Ti0.004 TiO₂ 0.007 Cr (ppm) <50 Sum 103.9

In one example of this process carried out in a fluidized-bed reactivecrystallizer, the pellets were grown up to approximately 0.85 mm to 1.2mm diameter in an alkaline environment containing approximately 0.5M to1.5M sodium hydroxide and approximately 0.2M to 0.6M sodium carbonate onaragonite seed material. The seed content was approximately 5-10 wt % ofthe mature pellet mass discharged from the bed. The data collectedshowed that the pellets had low pore volume (about 0.0009 to 0.0011cm³/g), small average pore diameter (about 6.54 to 6.65 nm) and very lowporosity surface area (about 0.40 to 0.48 m²/g).

One goal of fluidized-bed reactive crystallizers is to improve thecompetition of solid precipitation onto pellets versus spontaneousnucleation (e.g., fines production). Factors that impact this mayinclude the energy required for crystallization, relativesupersaturation, and speed at which CaOH₂ dissolves. For example, lessenergy may be required for crystallization onto pellets/seeds than forspontaneous nucleation, so feeding and properly mixing the inflowchemical (NaOH or Ca(OH)₂) into a bed of pellets/seeds helps to reducefines production. If the relative supersaturation is controlled andminimized at the feed point and/or where the calcium species dissociatesin the presence of carbonate (e.g., distributed throughout the bedrather than all at the bottom of the bed), spontaneous nucleation isless likely to occur and instead crystal growth on the solid bedmaterial is promoted. Further, if the speed at which CaOH₂ dissolves istoo fast, it can create problems with local calcium carbonatesupersaturation, especially in the entry zone.

Example implementations of systems and/or processes described hereinfeature the use of a fluidized-bed reactive crystallizer rather than astandard crystallizer vessel, and the process features a low carbonateconversion rate, tolerance for TSS in influent streams, high solution pHand slow dissolution of Ca(OH)₂ slurry.

The fluidized-bed reactive crystallizer may be different from a standardcrystallizer vessel in that the fluidized-bed's upward flow of liquidsuspends the solid bed and induces very high rates of mixing whileretaining a higher bed density than a standard crystallizer. The highrates of mixing are required to prevent local calcium carbonatesupersaturation at the point of Ca(OH)₂ addition which would lead tospontaneous nucleation and the production of small particles. The higherbed density results in significant solid bed mass surface area (e.g.,consisting of growing pellets and/or seed material) per unit volume. Theprecipitation of a solid onto a surface requires less energy and is thusfavored over spontaneous nucleation, but is often limited in standardcrystallizers by the low amount of surface area per unit volume.

The high bed density in the fluidized-bed reactive crystallizer providesa relatively larger surface area for the calcium carbonate toprecipitate onto, thus reducing the amount of spontaneous nucleationrelative to a system which does not have a bed of solid material. Thisenvironment promotes growth of individual pellets having a major axislength of up to approximately 2 mm instead of the much smaller particlesseen in standard crystallizers (major axis lengths of the individualparticles produced in standard crystallizers are in the 5 to 100 micronrange).

Example implementations of systems and/or processes described hereinfeature operation at higher effluent TSS levels, which are expected toproduce poor reactor performance in lower pH (e.g., pH 7-10) systems.This is because it is typically expected that a higher TSS in theeffluent is an indication that the calcium is nucleating and formingfines rather than precipitating on the bed mass and causing crystalgrowth, therefore retaining the calcium as part of the bed mass withinthe fluidized-bed reactive crystallizer. In example implementations ofsystems and/or processes described herein, the system can be operated atmuch higher effluent TSS, for example, between 500 ppm and 2000 ppm,rather than below 100 ppm as commonly seen in water softening and watertreatment applications. For example, the systems and processes describedherein may not follow the expected reduction in performance associatedwith high TSS content in the influent and instead may operate withhigher TSS levels while achieving economically viable retention rates atcalcium loading rates comparable to water treatment industries. Forexample, initial tests of the described systems and processes were runat much lower loading rates of about 2 kg-Ca/m²/hr in order to minimizeeffluent TSS to approximately 200 ppm in order, for instance, tomaintain the high retention levels of about 84% to 93% seen in theseinitial tests. However, tests showed unexpected results in that calciumretention above 85% at effluent TSS levels of ˜1400 ppm and a calciumloading rate of 15 kg-Ca/m²/hr were achieved. These results wereunexpected but show that, while having potentially high TSS, exampleimplementations of systems and/or processes described herein can stillwork at economically viable loading rates.

Example implementations of systems and/or processes described hereinoperate with high pH (for example, pH>14) where most of the CO₂ ispresent as carbonate (CO₃), and very little as bicarbonate (HCO₃). Otherapplications of fluidized-bed reactive crystallizers, such as seen inwater treatment, operate at lower pH of approximately 8 to 10, and assuch are dependent on the amount of bicarbonate and carbonate in thewater, due to the nature of the bicarb-carb equilibrium reaction. Thiseffects the relative calcium carbonate supersaturation within the vesselin a very different manner than when the pH is higher, for example above12, as in example implementations of systems and/or processes describedherein. Thus, the kinetics are different operating at the higher pH ofthe current application, and the governing resistances within theprocess are also different.

In some aspects depicted by FIG. 2, the systems and/or processes enablethe use of a simple reactor design, for example, a spouted or conefluidized-bed reactor. The slow dissolution rate of Ca(OH)₂ in exampleimplementations of systems and/or processes described herein allow forthe Ca(OH)₂ slurry to be added upstream of the solid bed mass, forexample mixed into the influent solutions (e.g., recirculation and/oralkaline solution) upstream of the fluidized bed material. The spoutedbed or cone fluidized-bed reactor design may be simpler, less expensive,and better suited for direct feed of a concentrated lime slurry whencompared to systems designed to deal with chemical addition inapplications where the chemical dissolution rates are much faster andwhere relative calcium carbonate supersaturation is more likely tooccur.

In some aspects, the use of influent solution that is low in impuritiesconsisting of, for example phosphates, phosphonates, Group II A ions,polyacrylic acid, iron, and magnesium, is preferred, as these impuritiescan inhibit the calcium carbonate growth rate or become included withinthe crystal structure, changing the crystal structure and properties. Insome implementations, Group II A ions may include ions of elements Be,Mg, Ca, Sr, Ba, and Ra on the periodic table of elements. These elementsare also often called alkaline-earth metals. These metals tend to losetwo electrons to form M₂+ ions, such as Ca₂+, Be₂+, Mg₂+, etc. Theamount of Group IIA ions brought into the process may be controlled,since these metal ions have the ability to compete with the intendedCa₂+ ions in the crystallization lattice causing changes to the pelletproperties. These ions can also react to form salt compounds and, ifincorporated into the pellet, may change the pellets physical and/orreactive properties, for example making the pellet softer and more proneto attrition. Other ions or additives could be selected which whenincluded in the crystal lattice via the method above, will result in anincrease in hardness of the macroscopic crystal, making it less prone toattrition.

Bivalent iron can attach itself into the calcium carbonate crystalstructure, and due to its size difference from calcium carbonate, it cancause disruptions to the crystal structure, placing a strain on thecrystal structure that leads to higher risk of attrition and fines.Phosphates can also act in a similar fashion, whereby they attach to thecrystal and prevent further. In addition, some phosphonate compoundssuch as HEDP have been shown to inhibit calcium carbonate growth (e.g.,used in the cooling tower/chiller industry to prevent scaling).

Additives (and for that matter impurities) may alter crystal nucleationand growth. Whether a given additive affects crystal nucleation (e.g.,rather than crystal growth) can be determined by measuring the width ofthe metastable zone, which is widened by nucleation inhibitors. Theremay be several generic features of how additives and impurities canaffect crystal nucleation. For example, to effectively inhibit crystalnucleation (e.g., fines formation), the additive should interactstrongly with the solute (in this case calcium carbonate solid) and alsohave a structure that can disrupt the periodicity associated with theemerging crystal nucleus. These characteristics may ensure that theadditive ends up within the aggregating crystals and that the crystalsmust disrupt their normal alignment in order to accommodate the additivemolecule. This may help to disrupt the nucleation process and hencefines formation. An additive like the type described above could beselected such that it has a greater affinity to be included within thecrystal structure of the calcium carbonate crystal aggregates thanalkalis and therefore will occupy the spaces within the crystalstructure which would normally be occupied by alkali.

The use of additives and/or operating conditions can also be used topromote growth of a certain shape of crystal, and/or a certain size ofcrystal which by its nature enforces a low porosity in the overallcrystal aggregate. Additive features such as those mentioned above canbe applied to the process described herein to make designer “fit forpurpose” additives, as well as to minimize impurities such that thecalcium carbonate pellets are produced as efficiently as possible (withminimum fines production) while still possessing optimum pelletproperties (e.g., low porosity, hardness, low alkali content) fordownstream processing.

In some aspects, additives may be added in a controlled fashion to alterphysical properties of components within the process or kinetics of theprocess. For example, in one application, an additive such asnitrilotriacetic acid (NTA), phenol, sucrose, NH₄Cl or H-EDTA may beadded in controlled amounts to increase the dissolution rate of Ca(OH)₂.Increasing the dissolution rate of Ca(OH)₂ would enable the use of ashorter reactor vessel, since a shorter residence time within the solidbed mass would be required to dissolve the Ca(OH)₂ and then react theCa₂+ ion to form CaCO₃.

In some aspects, an additive is added to the process in a controlledfashion to reduce the presence of low melting point salts contained withthe pellets. In some applications, such as where the calcium carbonatecrystal aggregates are sent to a calciner for further processing, thepresence of low melting point salts in the calcium carbonate can causefouling and operational issues in the calciner and downstream equipment.Examples of low melting point salts that may cause operational issues indownstream equipment are Na₂CO₃, K₂CO_(3,) NaOH, KOH, and chlorides.

In some aspects, an additive is added to the process in a controlledfashion to increase the hardness of the calcium carbonate crystalaggregates produced ,such that attrition within the fluidized-bedreactive crystallizer and other downstream equipment such as conveyorsis minimized.

In some aspects, an additive is added to the process in a controlledfashion to decrease the porosity of the calcium carbonate crystalaggregates produced, such that downstream dewatering and dryingprocesses become simplified.

In some aspects, an additive is added to the process in a controlledfashion to decrease the viscosity of the Ca(OH)₂ slurry produced fromthe slaking process, allowing for lime slurry concentrations of greaterthan 30 wt % to be transported from the slaking unit at operatingtemperatures between 10° C. to 90° C. As an example, caustic potash(e.g., potassium hydroxide or KOH) or caustic soda (e.g., sodiumhydroxide or NaOH) has a dispersant behaviour and when added to theslaking unit, it may act to reduce the Ca(OH)₂ slurry viscosity, thusenabling easier transportation.

FIG. 4 illustrates an example process 1100 which continuously grows andprocesses calcium carbonate crystal aggregates by reacting the incomingCa(OH)₂ slurry and the alkaline carbonate solution via thecausticization reaction (reaction (2) above) to deposit a portion of theprecipitated calcium carbonate (CaCO₃) onto the existing bed of solidscausing the solids to grow in volume as well as reducing the dissolvedcarbonate content and increasing the hydroxide content of the liquidstream. As illustrated, the example process 1100 includes a unit 1110(e.g., a slaker unit), a fluidized-bed reactive crystallizer 1120, aseparation and washing station 1130, and a control system 1108.

The first step in this example process reacts calcium oxide with waterto form calcium hydroxide in unit 1110 (e.g., an industrial lime slakeror lime hydrator coupled to a mixing tank wherein additional water ismixed with the Ca(OH)₂ produced from the hydrator) via reaction (1)above. The water used in this reaction can be either in the liquid orgaseous state. When the water is in a liquid state the reaction may becarried out in a mixed tank reactor, one example being industrial limeslakers, with an integral process of removing and disposing ofun-reactable contaminants as stream 1112.

Stream 1101 of water fed into this device serves two purposes, first aportion of the water is consumed by the reaction to form the calciumhydroxide via reaction (1) and second an excess of water must be addedto produce a transportable slurry of calcium hydroxide labelled asstream 1111. The properties of the calcium hydroxide slurry stream 1111are selected and controlled to meet the requirements for the subsequentprocessing units. If the water in stream 1101 delivered to unit 1110contains dissolved carbonate the produced calcium hydroxide willspontaneously react with these compounds, via reaction (2) above, in anuncontrolled manner to produce a precipitate of calcium carbonate. Thisprecipitate will consist of particles each having a volume less thanthat of a 100 micron diameter sphere which is undesirable for theprocess described herein.

To reduce this uncontrolled reaction, at least 10 times more moles ofcalcium oxide should be supplied to this processing step than the molesof dissolved carbonate entering with the water as a part of stream 1101.In some aspects there will be control of the size of the Ca(OH)₂particles produced in slaking unit 1110 and subsequently added to thefluidized-bed reactor 108. The control of Ca(OH)₂ particle size, and therelated specific surface area, is used to optimize the overall processperformance. Particle size can be controlled, either directly orindirectly, by the type of slaking system being used (e.g., a pasteslaker, detention slaker, ball mill slaker, batch slaker, hydratorsystem), by the use of additives such as chlorides or by the control ofoperating parameters associated with the slaking unit.

Operating parameters such as the slaking temperature, lime (CaO) towater ratio, degree of agitation during slaking, viscosity of theslurry, slaking time, and water temperature can all affect the particlesize, and/or the related specific surface area of the Ca(OH)₂ particlesproduced. The calcium carbonate supersaturation within the fluidized-bedreactive crystallizer may be controlled by the concentration ofdissolved calcium. At steady state operation, for example, the calciumhydroxide loading to the reactor 108 is fixed, within a range of about15kg-Ca/m²/hr and 30kg-Ca-m²/hr. As such, changing the size of theinfluent Ca(OH)₂ solid particles may change the related particle surfacearea, which may alter the solid-solution interface leading to eitherfaster or slower dissolution. This may also increase or decrease therisk of local supersaturation, respectively.

If the lime particles dissolve slower, this will decrease the risk ofsupersaturation but increase the required residence time in thefluidized solid bed mass to promote crystal growth onto the solid bedmass instead of into fines production outside of the bed. Larger Ca(OH)₂particles in the slurry may also increase the risk of Ca(OH)₂ solidssettling out of the slurry upstream of the fluidized-bed reactivecrystallizer (in the slaking system and associated piping).

Once the calcium hydroxide slurry (stream 1111) is produced it istransferred to unit 1120 which grows calcium carbonate crystalaggregates by reacting the calcium hydroxide slurry with the dissolvedcarbonate content in liquid stream 1106 in a controlled manner viareaction (2). This reaction is carried out in a fluidized-bed reactivecrystallizer, unit 1120, which has been optimized to operate in a highpH environment. The calcium hydroxide slurry of stream 1111 is meteredinto this device to control the rate at which calcium carbonate isformed and the device is arranged such that reaction (2) occurs in closeproximity to growing calcium carbonate crystal aggregates.

As a result of controlling the rate of reaction (2) and the environmentas reaction (2) occurs, calcium carbonate crystal aggregates, eachhaving an average volume equivalent to spheres with diameters between0.1 mm and 2 mm, are produced. The liquid from this equipment has had aportion of the dissolved carbonate precipitated out (reducing thedissolved carbonate concentration) and additional hydroxide dissolved in(increasing the dissolved hydroxide concentration), and the resultingliquid with its modified composition is discharged as the producthydroxide stream 1122 while the solid calcium carbonate crystalaggregates which have grown to the desired size are removed from thefluidized bed reactive crystallizer as a mixture of solution and calciumcarbonate crystal aggregates and sent onward to the next processing stepas stream 1121. Unit 1120 could be supplied with seed material tofacilitate the growth of the solid calcium carbonate crystal aggregates.The seed material could be made by crushing or grinding a portion of theproduced calcium carbonate crystal aggregates or supplied from anexternal source, such as limestone or sand or a designated seedfluidized-bed reactive crystallizer.

Due to the size, morphology, and physical properties of the calciumcarbonate crystal aggregates in stream 1121 they can be easily separatedfrom the solution in stream 1121, such that the accompanying solutionafter separation is less than 15% of the total weight of the calciumcarbonate crystal aggregates, in unit 1130 using industrial separationequipment such as screens or spiral classifiers. The calcium carbonatecrystal aggregates are then washed with stream 1103 which includes cleanwater and removes a majority of any residual solution on the calciumcarbonate crystal aggregate surface. The solution and water removed inthis processing step can both be delivered as part of the hydroxidestream 1131 or recycled back to the fluidized-bed reactive crystallizeror removed from the system altogether or delivered to the slaker as itsrequired water input given it has low enough carbonate content. Thecalcium carbonate crystal aggregates which have now been separated fromthe hydroxide solution are sent as stream 1132 to an end user.

Process 1100 also includes a control system 1108 that is communicablycoupled to at least one of the unit 1110 (e.g., slaker), the fluidizedbed reactive crystallizer 1120, the separation and washing station 1130,through communication elements 1115. Implementations of the controlsystem 1108 can include digital electronic circuitry, or computersoftware, firmware, or hardware, including the structures disclosed inthis specification and their structural equivalents, or combinations ofone or more of them. For example, the control system 1108 can be amicroprocessor based controller (or control system) as well as anelectro-mechanical based controller (or control system). Instructionsand/or logic in the control system (e.g., to control the process 1100 orother processes implemented by the unit 1110 (e.g., slaker), thefluidized bed reactive crystallizer 1120, the separation and washingstation 1130, can be implemented as one or more computer programs, e.g.,one or more modules of computer program instructions, encoded oncomputer storage medium for execution by, or to control the operationof, data processing apparatus. Alternatively or in addition, the programinstructions can be encoded on an artificially generated propagatednon-transitory signal, e.g., a machine-generated electrical, optical, orelectromagnetic signal that is generated to encode information fortransmission to suitable receiver apparatus for execution by a dataprocessing apparatus.

The control system 1108 can include clients and servers and/or masterand slave controllers. A client and server are generally remote fromeach other and typically interact through a communication network. Therelationship of client and server arises by virtue of computer programsrunning on the respective computers and having a client-serverrelationship to each other. In some aspects, the control system 1108represents a main controller (e.g., master) communicably coupled throughcommunication elements 1115 (e.g., wired or wireless) with each of theillustrated components of the process 1100.

FIG. 5 illustrates an example process 1200 for growing and processingcalcium carbonate crystal aggregates in a solution environmentconsisting of effluent (known as green liquor) from a pulp plant, and asa result of the causticization reaction occurring, reducing thedissolved carbonate content and increasing the dissolved hydroxidecontent of the liquid stream and delivering the resulting stream to thepulp plant as clarified white liquor. Process 1200, as illustrated,includes a pulp plant 1210 in material communication with the fluidizedbed reactive crystallizer 1120, and the separation and washing unit1130. Further, process 1200 includes, in the illustrated implementation,the unit 1110 (e.g., slaker), the dryer 1140, and calciner 1150

In the illustrated process 1200, the pulp plant unit 1210 uses clarifiedwhite liquor stream 1221 from fluidized bed reactive crystallizer 1120and clarified white liquor stream 1231 from unit 1130 to process or cookwood chips during the pulp making process. The caustic wash water stream1232 from separation and washing unit 1130 may be combined with theblack liquor in the pulp plant 1210 to produce carbonate rich greenliquor stream 1211 which is supplied to the fluidized bed reactivecrystallizer unit 1120. Therefore, in some aspects, the process providesat least clarified white liquor to the pulp plant, and in turn theprocess receives the carbonate rich green liquor from the pulp plant.

In some aspects, the slaking unit 1110 as employed in this embodimentoperates in the same or similar manner, using the same or similar inputmaterial streams and outputting the same or similar materials as shownin FIG. 1 and described in the first embodiment.

Once the calcium hydroxide slurry (stream 1111) is produced it istransferred to unit 1120 which grows calcium carbonate crystalaggregates by reacting the calcium hydroxide slurry with the greenliquor stream 1211 discharged from the pulp plant unit 1210. The calciumhydroxide reacts with the dissolved carbonate content of stream 1211 ina controlled manner via reaction (2). This reaction is carried out in afluidized bed reactive crystallizer which has been optimized to operatein a high pH environment. The calcium hydroxide slurry of stream 1111 ismetered into this device to control the rate at which calcium carbonateis formed and the device is arranged such that reaction (2) occurs inclose proximity to growing calcium carbonate crystal aggregates. As aresult of controlling the rate of reaction (2) and the environment asreaction (2) occurs, calcium carbonate crystal aggregates with averagevolumes equivalent to spheres with diameters between 0.1 mm and 2 mm areproduced. The liquid from this equipment has had a portion of thecarbonate precipitated out (reducing the dissolved carbonateconcentration) and additional hydroxide dissolved in (increasing thedissolved hydroxide concentration), and the resulting liquid with itsmodified composition is discharged back to the Pulp Plant unit 1210 asthe product clarified white liquor stream 1221 while the solid calciumcarbonate crystal aggregates which have grown to the desired size areremoved from the fluidized bed reactive crystallizer as a mixture ofsolution and calcium carbonate crystal aggregates and sent onward to thenext processing step as stream 1121.

Due to the size, morphology, and physical properties of the calciumcarbonate crystal aggregates in stream 1121 they can be almostcompletely separated from the solution in stream 1121, such that theaccompanying solution after separation is less than 15% of the totalweight of the calcium carbonate crystal aggregates, in unit 1130 usingindustrial separation equipment such as screens or spiral classifiers.The undiluted hydroxide solution is sent back to the Pulp Plant (unit1120) as stream 1231 and has similar properties to clarified whiteliquor in the pulping process. The calcium carbonate crystal aggregatesare then washed with stream 1103 which consists of clean water andremoves a majority of any residual solution on the calcium carbonatecrystal aggregate surface. The spent caustic wash water (stream 1232 )is also sent back to the Pulp plant (unit 1210), to be used to dissolvethe black liquor and form green liquor. The solution and water removedin this processing step could instead be recycled back to the fluidizedbed reactive crystallizer unit 1120. The calcium carbonate crystalaggregates which have now been washed and separated from the hydroxidesolution are sent as stream 1132 to a system such as a calciner which isdesigned to convert them from CaCO₃ to CaO which can be reused in theslaker unit 1110. The pellets produced by this process have physicalproperties and characteristics, such as size, low porosity, low alkalicontent, which are preferable to lime mud and enable the use of moreefficient and/or low cost calcination equipment.

FIG. 6 illustrates an example process 1500 for growing and processingcalcium carbonate crystal aggregates by reacting the incoming Ca(OH)₂slurry and the alkaline carbonate solution via the causticizationreaction (reaction (2) above) to deposit a portion of the precipitatedcalcium carbonate (CaCO₃) onto the existing bed of solids causing thesolids to grow in volume as well as reducing the dissolved carbonatecontent and increasing the hydroxide content of the liquid stream asapplied in the process of capturing CO₂ from atmospheric air. Process1500, as illustrated, includes a gas absorber or air-absorber orair-contactor 1510 in material communication with the fluidized bedreactive crystallizer 1120 and the separation and washing station 1130.Further, process 1500 includes, in the illustrated implementation, theunit 1110 (e.g., slaker), the dryer 1140, and calciner 1150.

In the illustrated process 1500, the gas absorber unit 1510 absorbs afraction of CO₂ from atmospheric air using a combination of CO₂-leansolution stream 1421 from the fluidized bed reactive crystallizer 1120and dilute CO₂-lean stream 1431 from separation and washing unit 1130.After the absorption of CO_(2,) the gas absorber returns the resultingCO₂-rich stream 1411 to unit 1120. Therefore, in one aspect, at leastCO₂-lean solution may be provided to the gas absorber, and the processin-turn receives the CO₂ rich solution from the gas absorber.

In the illustrated process 1500, in association with a calciner, CO₂capture, and a gas absorber is illustrated. In some aspects, the slakingunit 1110 as employed in this embodiment operates in the same or similarmanner, using the same or similar input material streams and outputtingthe same or similar materials as shown in FIG. 4 and described in thefirst embodiment.

Once the calcium hydroxide slurry (stream 1111) is produced it istransferred to unit 1120 which grows calcium carbonate aggregates byreacting the calcium hydroxide slurry with the CO₂-rich stream 1411discharged from the air or gas absorber unit 1510. The calcium hydroxidereacts with a portion of the dissolved carbonate content of stream 1411in a controlled manner via reaction (2). This reaction is carried out ina fluidized bed reactive crystallizer which has been optimized tooperate in a high pH environment. The calcium hydroxide slurry of stream1111 is metered into this device to control the rate at which calciumcarbonate is formed and the device is arranged such that the reaction toform the calcium carbonate occurs in close proximity to growing calciumcarbonate crystal aggregates.

As a result of controlling the rate of reaction (2) and the environmentas reaction (2) occurs calcium carbonate crystal aggregates with averagevolumes equivalent to spheres with diameters between 0.1 mm to 2 mm areproduced. The liquid from this equipment has had a portion of thecarbonate precipitated out (reducing the dissolved carbonateconcentration) and additional hydroxide dissolved in (increasing thedissolved hydroxide concentration), and the resulting liquid with itsmodified composition is discharged back to the air or gas absorber unit1510 as the CO₂-lean stream 1421 while the solid calcium carbonatecrystal aggregates which have grown to the desired size are removed fromthe fluidized bed reactive crystallizer as a mixture of solution andcalcium carbonate crystal aggregates and sent onward to the nextprocessing step as stream 1121.

Due to the size, morphology, and physical properties of the calciumcarbonate crystal aggregates in stream 1121 they can be almostcompletely separated from the solution in stream 1121, such that theaccompanying solution after separation is less than about 15% of thetotal weight of the calcium carbonate crystal aggregates, in unit 1130using industrial separation equipment such as screens or spiralclassifiers. The calcium carbonate crystal aggregates are then washedwith stream 1103 which includes clean water and removes a majority ofany residual solution on the calcium carbonate crystal aggregatesurface. Both the spent caustic wash water and the undiluted hydroxidesolution are mixed and sent back to the gas absorber unit 1510 as theCO₂-lean stream 1431. The solution and water removed in this processingstep can both be delivered back to the gas absorber unit 1510 orrecycled back to the fluidized bed reactive crystallizer unit 1120. Thecalcium carbonate crystal aggregates which have now been washed andseparated from the hydroxide solution are sent as stream 1132 to adrying unit 1140 which vaporizes any residual moisture.

The calcium carbonate crystal aggregates in this process carry verylittle water on their surface after they are separated from solution inunit 1130 and are then processed by drying unit 1140 to remove thisresidual moisture. The residual moisture, in either liquid or vapourphase, is discharged from the process as a portion of stream 141. Due tothe size and morphology of the crystal aggregates, fluidized bed dryerscan be employed as unit 1140 which can make use of heat supplied usingadvanced drying processes like super-heated steam dryers, vapourrecompression dryers, and in bed heat exchange tubes. Specifically,fluidized bed dryers could operate on low grade heat which could bebelow or only slightly above 100° C., hot gases from other points in theprocess, or in the case of vapour recompression systems, electricalenergy drives a heat pump which could deliver up to 60 kJ of heat byconsuming 1 kJ of electricity. Alternatively, the dryer 1140 may be acontact dryer such as, for example, vacuum tray, vertical agitated,double cone, horizontal pan, plate, vacuum band, horizontal, paddle orindirect rotary dryers. The dryer 1140 may also be a dispersionconvective dryer other than a fluidized bed dryer, such as spouted bed,direct rotary and pneumatic conveying dryers, or layer convective dryerssuch as convective tray, through-circulation, turbo-tray, tunnel, movingbed, paddle, or a rotary-louver dryer.

In addition to the calcium carbonate crystal aggregates in stream 1132,stream 1105 of make-up calcium carbonate is introduced into drying unit1140 to account for losses of calcium compounds throughout the process.The source of make-up calcium carbonate could be limestone. Stream 142of dry calcium carbonate crystal aggregates and make-up calciumcarbonate are now ready for the final processing step.

The calcium carbonate crystal aggregates are then reacted to reform thecalcium oxide which was used in the first step in the process andrelease a gas stream containing CO₂ via reaction (3) above. Thisreaction takes place at approximately 900° C., requires heat energy asan input, and is carried out in unit 1150, commonly known as calciner.The heat could be supplied to the calciner by the combustion ofhydrocarbons such as natural gas, fuel oil, or coal, biomass, the use ofsolar heat, electricity, or a combination thereof

The calciner (e.g., unit 1150) employed could be a rotary kiln, shaftkiln, flash calciner, or fluidized bed calciner. In some embodiments,the necessary heat is supplied when stream 104 of fuel is combusted withthe oxygen in gas stream 102 which could consist of air or oxygen froman air separation unit (ASU). The products from combustion of fuel andthe CO₂ from reaction (3) mix together and are discharged from calciningunit 1150 as off-gases 1152. The washing previously described and onlyachievable with the calcium carbonate crystal aggregates enables the useof fluidized bed calciners, which are desirable because they areinherently easy to control and have very uniform internal conditions,both are factors which lead to more reactive CaO in stream 1151 withless material which has been over heated, over-burnt or sintered and ofwhich must be disposed. Stream 1151 is returned to unit 1110 such thatthe process can be repeated. Stream 1152 of off-gases are hot and couldbe used to preheat streams 102, 104, or 142 before they enter thecalcination unit 1150, as the heat source for drying unit 1140, or togenerate steam in a boiler. Stream 1152 can be applied to these variousprocesses because a majority of the solution was removed in unit 1130,and the size and morphology of the calcium carbonate crystal aggregatesprevents contamination of stream 1152 with unmanageable quantities ofdust from stream 142 or steam 1151 or chemicals from stream 1121. Oncethe heat in stream 1152 has been used as described above the resultantgases could be delivered to a consumer of CO₂ enriched air such as agreenhouse or algaculture facility or in the case where stream 102consisted of oxygen from an air separation unit stream 1152 is primarilyCO₂ and can be delivered to a user of CO₂ such an enhanced oil recoveryfield.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. For example,advantageous results may be achieved if the steps of the disclosedtechniques were performed in a different sequence, if components in thedisclosed systems were combined in a different manner, or if thecomponents were replaced or supplemented by other components. Further,in some implementations, one or more processes disclosed here, may beperformed with additional steps, fewer steps, or may be performed indifferent orders than those disclosed herein, within the scope of thepresent disclosure. As another example, although a control system (e.g.,control system 1108) is not illustrated as part of every disclosedprocess and/or system, each of the aforementioned processes (e.g.,systems 200 and 300 and otherwise) may include a control system or acontroller (e.g., similar to controller 110 and/or control system 1108)communicably coupled to the components (illustrated or otherwise) andconfigured to perform operations and/or execute instructions toimplement such processes (and other processes). Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A method for growing crystalline calciumcarbonate solids in the presence of an alkaline carbonate solution in afluidized-bed reactive crystallizer the method comprising: reacting, ina slaking process, quicklime (CaO) and a low carbonate content fluid toyield a slurry of primarily slaked lime (Ca(OH)₂); introducing theslurry of primarily slaked lime and an alkaline solution comprisingbetween 0.1M to 4.0 M hydroxide and between 0.1 M to 4.1M carbonate intoa fluidized-bed reactive crystallizer comprising a bed of solidparticles flowing the slurry of primarily slaked lime and the alkalinesolution through the fluidized-bed reactive crystallizer to fluidize thebed of solid particles, the slurry of primarily slaked lime and thealkaline solution having a fluidization flow velocity within a range of2 m/h to 70 m/h; reacting the Ca(OH)₂ slurry and the alkaline carbonatesolution to deposit a portion of precipitated calcium carbonate (CaCO₃)onto the bed of solid particles; and discharging a portion of the bed ofsolid particles and the alkaline solution.
 2. The method of claim 1,wherein discharging a portion of the bed of solid particles and thealkaline carbonate solution includes discharging one or more solidparticles with a diameter from 0.1 mm to 0.4 mm.
 3. The method of claim2, wherein: discharging a portion of the bed of solid particles and thealkaline solution includes discharging one or more solid particlescomprising a volume from 0.0005 mm³ to 0.035 mm³.
 4. The method of claim1, wherein introducing the slurry of primarily slaked lime comprisesinjecting the slurry into a recirculation stream influent upstream ofthe fluidized-bed reactive crystallizer.
 5. The method of claim 4,wherein introducing the alkaline solution comprises injecting thealkaline solution into the recirculation stream influent upstream of thefluidized-bed reactive crystallizer.
 6. The method of claim 1, furthercomprising introducing an additive.
 7. The method of claim 6, whereinintroducing an additive includes introducing an additive comprising atleast one of: nitrilotriacetic acid, phenol, sucrose, NH₄Cl, and H-EDTA.8. The method of claim 1, further comprising extending a height of thebed of solid particles to between 15 feet and 50 feet.
 9. The method ofclaim 1, further comprising controlling a level of total suspendedsolids within the fluidized-bed reactive crystallizer to between 100 ppmand 1500 ppm.
 10. The method of claim 1, wherein discharging a fractionof the bed of solid particles and the alkaline carbonate solutionincludes discharging a set of solid particles comprising a diameterwithin a range of 0.1 mm to 0.2 mm.
 11. The method of claim 1, furthercomprising increasing a volume of the solid particles by reacting theCa(OH)₂ slurry and the alkaline carbonate solution to deposit a portionof the precipitated calcium carbonate (CaCO₃) onto the bed of solidparticles.
 12. The method of claim 1, further comprising decreasing aconcentration of dissolved carbonate by reacting the Ca(OH)₂ slurry andthe alkaline carbonate solution to deposit a portion of the precipitatedcalcium carbonate (CaCO₃) onto the bed of solid particles.
 13. Themethod of claim 1, increasing a concentration of dissolved hydroxide byreacting the Ca(OH)₂ slurry and the alkaline carbonate solution todeposit a portion of the precipitated calcium carbonate (CaCO₃) onto thebed of solid particles.
 14. A method for growing crystalline calciumcarbonate solids in the presence of an alkaline carbonate solution in afluidized-bed reactive crystallizer, the method comprising: introducinga slurry of primarily slaked lime and an alkaline solution comprisingbetween 0.1M to 4.0 M hydroxide and between 0.1M to 4.1M carbonate intoa fluidized-bed reactive crystallizer comprising a bed of solidparticles; and adding the slurry of primarily slaked lime to thefluidized-bed reactive crystallizer at a loading rate between 5kg-Ca/m²/h to 35 kg-Ca/m²/h.
 15. The method of claim 14, whereinintroducing the slurry of primarily slaked lime and an alkaline solutioninto a fluidized-bed reactive crystallizer includes introducing theslurry of primarily slaked lime at one or more entry points.
 16. Themethod of claim 14, wherein introducing the slurry of primarily slakedlime comprises injecting the slurry into a recirculation stream influentupstream of the fluidized-bed reactive crystallizer.
 17. The method ofclaim 16, wherein introducing the slurry of primarily slaked lime andthe alkaline solution comprises injecting the alkaline solution into therecirculation stream influent upstream of the fluidized-bed reactivecrystallizer.
 18. The method of claim 14, further comprising introducingan additive.
 19. The method of claim 18, wherein introducing an additiveincludes introducing an additive comprising at least one of:nitrilotriacetic acid, phenol, sucrose, NH₄Cl, and H-EDTA.
 20. Themethod of claim 14, further comprising extending a height of the bed ofsolid particles to between 15 feet and 50 feet.
 21. The method of claim14, further comprising controlling a level of total suspended solidswithin the fluidized-bed reactive crystallizer to between 100 ppm and1500 ppm.
 22. The method of claim 14, further comprising reacting, in aslaking process, quicklime (CaO) and a low carbonate content fluid toyield the slurry of primarily slaked lime, wherein reacting quicklimeand a low carbonate content fluid includes supplying 10 moles ofquicklime (CaO) per 1 mole of dissolved carbonate.
 23. The method ofclaim 22, wherein supplying 10 moles of CaO per 1 mole of dissolvedcarbonate to a slaker reduces a portion of solid particles in thefluidized-bed reactive crystallizer having a volume less than a volumeof a 100 um diameter sphere.